Aqueous solutions containing hypochlorite ions, ClO!.sup.-, are corrosive to many metals and are highly toxic to aquatic life. Hypochlorite-containing solutions which are evolved as waste streams or side-products require treatment to remove or destroy the deleterious hypochlorite ions before the aqueous stream can be released into rivers, bays or other public waters. Aqueous waste streams containing hypochlorite ions are produced, for example, by chlor-alkali production facilities.
Various methods are known for destroying hypochlorites, but they are too inefficient or too expensive for large scale applications involving very large quantities of aqueous streams which contain hypochlorites.
Thermal decomposition may be used in some cases, but when large quantities of dilute hypochlorites require decomposition, the cost of heat (energy) becomes prohibitive, and the rate of decomposition is inefficiently slow. In addition to the cost of the heat, it is necessary in some instances to cool the aqueous stream before it reaches public waters in order to avoid "thermal pollution". Cooling the aqueous stream requires extra handling and energy consumption and therefore, increased costs.
Actinic radiation (light) accelerates the decomposition, but this requires either using large glass vessels (impractical) or the use of light sources inside opaque vessels and is inefficiently slow for large scale use.
Hypochlorite is chemically reactive, but proposed reactants are either expensive (e.g., H.sub.2 O.sub.2), tend to produce deleterious side products (e.g., NaHS), or require concentrated solutions (e.g., HCl). Usually the reaction product would require recovery because of ecological or economical reasons.
It is well known that certain transition metal ions (e.g., from transition metal salts) catalyze the decomposition of hypochlorite to chloride ion plus molecular oxygen. However, practical utilization of this knowledge has been hindered by the reactivity and/or solubility of these transition metal ions in the solutions in which hypochlorite is likely to be found. It has been proposed that soluble transition metal salts be added to waste streams to decompose hypochlorite, but this would require an expensive and complex recovery step to prevent loss of the expensive transition metal ion and would risk the consequent pollution of the waste stream by the heavy metals.
In an application filed Dec. 12, 1974 as Ser. No. 531,934, now U.S. Pat. No. 3,977,958, which is a continuation-in-part of Ser. No. 425,410 filed Dec. 17, 1973, now abandoned, it is shown that an electroconductive substrate having a coating thereon of a single-metal spinel structure of Co.sub.3 O.sub.4 is useful as an electrode in an electrolytic process. There is no suggestion that the Co.sub.3 O.sub.4, if used as a catalyst instead of as an electrode, would promote the decomposition of aqueous hypochlorites.
There is a need for an efficient, economically feasible method for decomposing large quantities of dilute hypochlorite.
It is an object of the present invention to provide an efficient, commercially feasible method for destroying hypochlorites in aqueous streams.
A further object is to provide a means for destroying hypochlorites in aqueous streams without adding further chemicals to the aqueous stream which, for economical or ecological reasons, would require subsequent removal before the treated stream is released into public waters.
Another object is to destroy hypochlorites in aqueous streams without expending wasteful amounts of heat energy and without unduly risking thermal pollution of public waters.
Yet another object is to provide a catalyst for use in the present inventive method which is efficient, is non-polluting and is long-lived.
These and other objects are attained by the invention disclosed hereinafter. Variations in the embodiments described herein will become apparent to practitioners of the pertinent art without departing from the invention claimed.